Memory expansion is one of the many fields where high density circuit module solutions provide space-saving advantages. For example, the well-known DIMM (Dual In-line Memory Module) has been used for years, in various forms, to provide memory expansion. A typical DIMM includes a conventional PCB (printed circuit board) with memory devices and supporting digital logic devices mounted on both sides. The DIMM is typically mounted in the host computer system by inserting a contact-bearing edge of the DIMM into a card edge connector. Typically, conventional DIMM-based solutions have typically provided only a moderate amount of memory expansion.
As bus speeds have increased, fewer devices per channel can bc reliably addressed with a DIMM-based solution. For example, 288 ICs or devices per channel may be addressed using the SDRAM-100 bus protocol with an unbuffered DLMM. Using the DDR-200 bus protocol, approximately 144 devices may be addressed per channel. With the DDR2-400 bus protocol, only 72 devices per channel may be addressed. This constraint has led to the development of the .fully-buffered DIMM (FB-DIMM) with buffered Command Address (C/A) and data in which 288 devices per channel may be addressed. That buffering function is provided by what is typically identified as the Advanced Memory Buffer or AMB. With the FB-DIMM, not only has capacity increased, pin count has declined to approximately 69 signal pins from the approximately 240 pins previously required.
The FB-DIMM circuit solution is expected to offer practical motherboard memory capacities of up to about 192 gigabytes with six channels and eight DIMMs per channel and two ranks per DIMM using one gigabyte DRAMs. This solution should also be adaptable to next generation technologies and should exhibit significant downward compatibility. The FB-DIMM solution does, however, generate significant thermal energy, particularly about the AMB.
There are several known methods to improve the limited capacity of a DIMM or other circuit board. In one strategy, for example, small circuit boards (daughter cards) are connected to the DIMM to provide extra mounting space.
In another strategy, multiple die package (MDP) can also be used to increase DIMM capacity. This scheme increases the capacity of the memory devices on the DIMM by including multiple semiconductor die in a single device package. The additional heat generated by the multiple die typically requires, however, additional cooling capabilities to operate at maximum operating speed. Further, the MDP scheme may exhibit increased costs because of increased yield loss from packaging together multiple die that are not fully pre-tested.
Stacked packages are yet another way to increase module capacity. Capacity is increased by stacking packaged integrated circuits to create a high-density circuit module for mounting on the larger circuit board. In some techniques, flexible conductors are used to selectively interconnect packaged integrated circuits. Staktek Group L.P., the assignee of the present application, has developed numerous systems for aggregating CSP (chipscale packaged) devices in space saving topologies. The increased component height of some stacking techniques may, however, alter system requirements such as, for example, required cooling airflow or the minimum spacing around a circuit board on its host system.
Typically, the known methods for improved memory module performance or enlarged capacity raise thermal management issues. For example, when a conventional packaged DRAM is mounted on a DIMM, the primary thermal path is through the balls of the package into the core of what is typically an epoxy based FR4 board that has less than desirable thermal characteristics. In particular, when an advanced memory buffer (AMB) is employed in an FM-DIMM, a significant amount of heat is generated. Consequently, the already marginal thermal shedding attributes of DIMM circuit modules is exacerbated in a typical FB-DIMM by the localized generation of heat by the AMB.
Memory DIMMs, both buffered and unbuffered, are often employed on motherboards mounted in server racks with limited space. Large capacity memory devices often have dimensions that create addition height issues (in the longitudinal direction away from the mounting socket).
What is needed, therefore, are methods and structures for providing high capacity circuit boards in thermally-efficient, reliable designs, that provide in some modes, the opportunity for concomitant reduction in module height.